1) Field of the Invention
The present invention relates to a long magnet, its production method, a magnet roller and an image forming device.
2) Description of the Related Art
In the image forming devices such as electrophotographic-type copy machines, printers, facsimiles, and multifunction peripherals, developing device have been widely used to develop a latent image formed on an image carrier with the use of a developer made of two components of toner and carrier. Such a developing device develops the latent image in the following manner. That is, forms a magnetic brush by magnetic adsorption of a developer to the outer peripheral surface of a developing roller, and selective supplies and adheres toner to a latent surface of an image carrier that is facing the magnetic brush by an electric field between the image carrier where an electrostatic latent image is formed and a sleeve applied with electrical bias in a developing region (a region where an electric field capable of development between the developing roller and the image carrier is secured).
As disclosed in Japanese Patent Application Laid-Open Publication No. 2001-296744, the developing rollers require high magnetic characteristic for magnet materials due to a small angle between the poles in the developing polar portion. Moreover, the accuracy of the developing polar portion is required to be high.
These requirements cannot be fulfilled with the known materials or know structures of the rollers. For example, the ferrite-type magnets do not have sufficient magnetic characteristic. The rare earth magnets have high magnetic characteristic; however, they are costly. One approach is to use the rare earth magnet only for the developing pole, which requires high magnetic characteristic, and use the ferrite-type magnet for other poles. To meet the requirements described above, a magnet block is made of rare earth magnet and fit in a groove formed on a cylindrical plastic magnet to form a magnet roller, and then the magnet roller is used for, the developing roller. It should be noted that the magnet block can be made by sintering, extrusion molding, injection molding or compression molding.
However, 100 millimeters is generally a limit for the length of a sintered magnet block made of rare earth. It is difficult to make a 300 millimeters long magnet block that is used for a developing roller. A 300 millimeters long magnet block can be made by extrusion molding or injection molding; however, achieving uniform accuracy of dimension covering 300 millimeters without torsion or deflection is difficult. Furthermore, due to the molding characteristics, fluidity is necessary to some extent, and this causes an increase in proportion of binder and resin, and it is difficult to enhance the magnetic characteristic by increasing the content of magnetic powder in the magnet block. Therefore, a long magnet block is not easy to obtain even by sintering, extrusion molding or injection molding.
On the other hand, since a high magnetic force is achieved in a magnet block made by compression molding, it is possible and advantageous to increase the content of magnetic powder in the magnet block. There is, however, a problem that such a magnet block has a poor mechanical strength, particularly, flexural strength. Although a magnet block formed by compression molding is preferable in view of the magnetic force, the magnet block does not have enough strength required for delivery after having been taken out from the mold and a series of processes such as fitting of the magnet block in a groove of cylindrical plastic magnet, resulting in cracks and damage that were easy to occur.